How hot was it? July broke U.S. record

With an average temperature of 85.4 degrees, July was the seventh-hottest in Memphis history. The record average of 88.8 degrees dates to the infamous summer of 1980, which people still talk about it as one of the worst ever. Average temperatures run higher in sultry Memphis than the entire United States, where the July average was 77.6 degrees, a record high for the lower 48 states.

It's official. July was the hottest month in the lower 48 states since the government began keeping temperature records in 1895.

The average temperature last month was 77.6 degrees — 3.3 degrees above the average 20th-century temperature, the National Oceanic and Atmospheric Administration reported on Wednesday. July thereby dethroned the record of 77.4 degrees set in July 1936, the agency said.

Climatologists at the agency noted that by the end of the month, about 63 percent of the nation was experiencing drought conditions, which contributed to the sweltering temperatures.

"July was a pretty interesting month because there were two different things at play," said Jake Crouch, a climatologist at the agency's National Climatic Data Center in Asheville, N.C.. "We saw very warm daytime temperatures over a large part of the country related to the ongoing drought, just as in 1936. When soils are dry, especially during the summer, it drives the daytime temperatures up. But this is a very local effect.

"On the other side, at the national level, we have also seen very warm nighttime temperatures, and that is part of a long-term trend we've seen across

the contiguous U.S. over the past several decades. The hotter days increase the amount of moisture the lower atmosphere can hold, and this means it doesn't cool off as much at night anymore."

"This clearly shows a longer-term warming trend in the U.S., not just one really hot month," Crouch said.

Three of the nation's five hottest months on record have been recent Julys: This year, 2011 and 2006. Julys in 1936 and 1934 round out the top five.

Last month also was 3.3 degrees warmer than the 20th century average for July.

Thirty-two states had months that were among their 10 warmest Julys, but only one, Virginia, had the hottest July on record. Crouch said that's a bit unusual, but that it shows the breadth of the heat and associated drought.

For example in 2011, the heat seemed to be centered mostly in Oklahoma and Texas. But this summer "the epicenters of the heat kind of migrated around. It kind of got everybody in the action this month," Crouch said.

The extreme temperatures are certain to stoke the debate about greenhouse emissions and their impact on the climate.

NASA scientist James Hansen reported in a journal article published Monday that recent extremes of hot weather have been so pronounced that scientists can say with near-certainty that events like the Russian heat wave of 2010 or the Texas heat wave of 2011 would not have happened unless global warming was under way. While some scientists praised his results, others argued that the evidence was still insufficient for attributing a specific heat wave or storm to human-induced climate change.

A vast majority of scientists agree that such events will become ever more common as the planet warms, however.

In the U.S., the only hope for substantial relief from warmer-than-average temperatures in the coming weeks and months would be a striking atmospheric change, like an El Nino.